The state of Mississippi's history extends to thousands of years of indigenous peoples. Evidence of their cultures has been found largely through archeological excavations, as well as existing remains of earthwork mounds built thousands of years ago. Native American traditions were kept through oral histories; the Europeans recorded accounts of historic peoples they encountered. Since the late 20th century, there have been increased studies of the Native American tribes and reliance on their oral histories to document their cultures. Their accounts have been correlated with evidence of natural events.